Constant load voltage circuit



1955 R. H. HAGOPIAN 2,727,212

CONSTANT LOAD VOLTAGE CIRCUIT Filed Oct. 27. 1950 Fig. I

Fig.2.

Fig. 3. Fig. 4.

24 24 Die'ec'rric Dieleciric Material Material WITNESSES: INVENTOR & Richard H. Hcgopicn.

ATTORN EY United States Patent Ofiice 2,727,212 Patented Dec. 13, 1955 CONSTANT LOAD VOLTAGE CIRCUIT Richard H. Hagopian, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Application October 27, 1950, Serial No. 192,423

3 Claims. (Cl. 333-27) My invention relates to high frequency heating applications and, more particularly, to the provision of a constant load voltage for primarily dielectric heating applications.

The present application is a continuation in part of a ccpending application of the same inventor filed on April 30, 1949, Serial No. 90,683, now abandoned.

in dielectric heating applications, there is a tendency for the load impedance to vary considerably due to the effects of the electrostatic field on the load material. Also, load impedance variation may be caused, especially in conveyor-fed apparatus adapted for mass production heat treatment, by variation of the material on the conveyor between a maximum and a minimum physical dimension. Further variation of the load impedance may be due to the number of workpieces on the conveyor which is feeding the heat treatment apparatus. Under such conditions, it is highly desirable to have the electrode voltage remain reasonably constant as the load varies from effective full load to no load.

The usual method of maintaining a relatively constant load voltage is by means of an automatic control circuit, which varies one or more of the load circuit tuning elements. Such an automatic control circuit is troublesome in operation, and further requires technically trained personnel for its maintenance and adjustment. The latter is a serious disadvantage since the users of industrial dielectric heating equipment may not have technically trained men available to do this work.

Accordingly, it is an object of my invention to provide apparatus and a method of maintaining a substantially constant load voltage regardless of load variation.

It is another object of my invention to make unnecessary the use of the prior art automatic control circuits which vary the load circuit tuning elements.

It is still another object of my invention to provide a circuit for dielectric heat treatment applications which will require no network adjustment during its normal course of operation.

It is a further object of my invention to provide a relatively inexpensive coupling circuit between the dielectric load electrodes and the power source.

In accordance with my invention, the load circuit is parallel tuned to a relatively high impedance. However, it is tuned away from parallel resonance, and on the inductive side of parallel resonance, by an amount such that during the normal operation of the dielectric heating apparatus and throughout the normal variation of the load impedance thereby, the load circuit remains on the inductive side of parallel resonance and does not either reach parallel resonance or cross over onto the capacitive side of parallel resonance. This relatively high impedance load circuit is connected to the receiving end of an effectively quarter-wave-length transmission line, such that the reflected load impedance or the sending-end irnpedance of said transmission line has a relatively low magnitude. The sending end of said transmission line is connected to a source of radio-frequency oscillations through a coupling circuit. The latter coupling circuit is tuned away from series resonance such that the effective impedance of said coupling circuit as presented to the source of oscillations is relatively high as compared to the sending-end impedance of the transmission line. In this manner, any changes in the sending-end impedance, resulting from variations in the load impedance due to the effect of the dielectric heating on said load do not materially affect the net series impedance of the coupling circuit. Therefore, the current of said coupling circuit remains substantially constant. The sending-end impedance of the transmission line is efiectively a series component of said coupling circuit and, therefore, the coupling circuit current is the sending-end current of the transmission line. By choosing an effectively quarter- Wave-length transmission line, the sending-end current of the transmission line determines the receiving-end voltage and, hence, the voltage across the load circuit. By so maintaining the sending-end current substantially constant, the receiving-end voltage and, hence, the load voltage is maintained substantially constant.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in conjunction with the accompanying drawing, in which:

Figure 1 is a schematic diagram of the over-all dielectric heating system.

Figure 2 is an equivalent circuit for a portion of the system shown in Figure 1, and

Figures 3 and 4 illustrate the variations in characteristics which the load material between the electrodes is likely to undergo.

In Figure 1 is shown a generator coupling circuit in accordance with my invention in which a radio-frequency generator 10 is coupled through a variable transformer 12 to the input or sending end of a transmission line 14. A coupling capacitor 16 is provided in series with the secondary winding 18 of the variable coupling transformer 12. A shunt capacitor 24} is connected across the primary winding 22 of the coupling transformer 12. One conductor of the transmission line 14 is grounded. The output or receiving end of the transmission line 14 has connected across it the dielectric heating electrodes 24 with a workpiece 26 placed between the electrodes 24. A variable tuning inductance 28 is connected in parallel with the load electrodes 24.

In Figure 2 is shown an equivalent load circuit for the radio-frequency generator coupling network of Fig. l. The effective impedance 30 of the coupling transformer 12 is shown in series with the resulting coupling capacitor 32 and the resultant sending-end impedance 34 of the transmission line 14.

in Figures 3 and 4 are shown respectively a load of small capacitive reactance and a load of greater capacitive reactance between the load electrodes 24.

In the operation of dielectric heat treatment apparatus where the quantity of workpieces actually undergoing heat treatment varies, the effective Q of the load circuit changes appreciably. As the load decreases, which is illustrated by the load as shown in Figure 4 compared to the load shown in Figure 3, the efiective capacitive reactance of the load combined with the load electrodes increases, and the efiective load resistance decreases to result in an increase of over-all load Q. The load Q is numerically equal to the load reactance X over the load resistance R in accordance with the following equation:

X load Q fie load For a given spacing between the load electrodes 24, as the quantity of load material decreases, there results a decrease of the effective load capacitance and, accordingly, an increase of the effective capacitive reactance. As the quantity of the load material decreases, the resistance thereof decreases. Therefore, the Q of the load increases due to an increase in the load reactance accompanied by a decrease in the load resistance.

If the load circuit be considered as comprising the parallel-tuned network, including the load material and the load electrodes shunted by the variable inductance, the net impedance of the load circuit has a maximum value when the load circuit is tuned to parallel resonance. To illustrate the effect of the load Q on the load circuit impedance, as the load Q increases, the parallel resonant impedance of the load circuit increases. As will be explained later, there is a compensating or balancing relationship between the effect of load Q variation on the load circuit impedance and the effect of load variation on load circuit impedance when the load circuit is tuned to the inductive side of parallel resonance and the load capacitance decreases.

Referring to Figure l, the receiving-end impedance of the transmission line 14 is equivalent to the load circuit impedance described in the above paragraph. It is characteristic of an effectively quarter-wave-length transmission line that the sending-end impedance ZS or the reflected load impedance, as presented at the input terminals of the transmission line, is a function of the characteristic impedance Z of the transmission line and the receiving-end impedance Zn. The following equation illustrates the latter relationship:

Therefore, if the impedance of the parallel-tuned load circuit is relatively high (for example, approximately equal to or greater than twice the surge impedance of the transmission line), the sending-end impedance ZS of the line 14 is always a relatively small value in magnitude.

Referring to Figure 2 where the sending-end impedance ZS of the transmission line 14 is schematically shown by a block diagram 34, which is in series with the coupling capacitor 32 and the effective impedance 38 of the transformer coupling between the radio-frequency power supply generator and the transmission line 14, if the equivalent circuit as shown in Figure 2 is series tuned away from series resonance, the resultant impedance of this circuit will be relatively high as compared to the sending-end impedance ZS. acteristic of a series resonant circuit that at the point of series resonance the impedance reaches a minimum value, and the actual impedance of the series circuit is proportionally greater as a function of the degree or amount away from the series resonant point that the circuit is tuned.

The theory behind my invention may be evolved as follows. The general equation for the voltage at the receiving end cf 2. transmission line in terms of the sending-end current and voltage, assuming a lossless transmission line, is as follows:

ER ES cos fil+jIsZa sin 81 where ER is the voltage at the receiving end of the transmission line, E5 is the voltage at the sending end of the transmission line, ,8 is a propagation constant, I is the length of the transmission line, i is an imaginary term equal to /Z, ls is the sending-end current of the transmission line. and Z0 is the characteristic impedance of the transmission line. By making the transmission line It is charapproximately a quarter-wave-length long, the receivingend voltage ER becomes 'a function of Is because the cosine of B1 approaches the cosine of and hence becomes zero. The receiving-end voltage Ea, which is the voltage across the load circuit, may be kept relatively constant, even with a varying load impedance Zn, if by some means the sending-end current Is is maintained constant.

To maintain Is constant, the dielectric load 26 with the electrodes 24, with its shunt variable inductance 23, is parallel tuned to present an effectively high impedance to the receiving end of the transmission line 14. It is preferable that the load circuit be tuned on the inductive side of its parallel resonance point so that as the load capacitance changes the circuit does not at any time either become parallel resonant or pass through parallel resonance and on to the capacitive side of resonance. For example, as the load capacitance decreases (which is likely under normal operation), the load circuit departs further away from the .parallel resonant point. As previously explained, such a decrease in the load capacitance is accompanied by a decrease of the efiective load resistance, and, therefore, the overall load circuit Q is increased. The net result is that the increased load circuit Q compensates for the change in load circuit impedance to in effect partially balance the latter, and the resultant and effective load'circuit impedance cha ge is less than would be caused if the load circuit were tuned to the capacitive side of the parallel resonant point.

if the load circuit impedance is kept reasonably high (in the order of twice the surge impedance of the line or greater), the effective impedance at the sending end of the transmission line 14 is always a relatively small value. T he latter may be readily understood with the help of the equation for the sending-end impedance of the quarter-wave-length transmission line (see Equation 2 above).

With the value of the sending-end impedance Z5 kept relatively small due to the load impedance reflection characteristics of the transmission line 14, the generator coupling circuit may be so tuned, as previously described, that the sending-end current Is of the transmission line Ll is relatively independent of the sending-end impedance Z5 over the range of values necessary for the operation of the described dielectric heating apparatus. A reference to the equivalent circuit shown in Figure 2 will be helpful in explaining the latter concept. The sending-end impedance 34 of the transmission line 14 is effectively a series component of the coupling circuit. The rest of the circuit is made up of the effective impedance 35) of the coupling transformer and the impedance 32 of the coupling capacitor. If this series circuit is tuned away from series resonance and, therefore, away from the series resonant minimum effective impedance, the net circuit impedance can be made relatively high compared with the sending-end impedance ZS of the transmission line 14, and very high relative to any reasonable changes in the sending-end impedance Z5 due to changes in the load per se. That is to say, the following relative circuit conditions should be maintained: j.rijxe+Zs Z8 the change of ZS.

due to normal changeof load impedance (4) Therefore, the sendin -end current Is will remain relatively constant and unaffected by any changes-in the characteristics of the load circuit per se. Since the coupling circuit is operated off series resonance, the generated voltage supplied by the radio-frequency power supply generator 16 must be large enough tosupply the required sendingcnd current Is.

Although the load circuit is parallel tuned to increase the effective load circuit. impedance and, therefore, the impedance at the receiving end. of the transmission line Zn, it is not desirable to tune the load circuit completely to parallel resonance. There are several good reasons for this, one being that any change of load can cause the load circuit to go to either side of the parallel resonant point. Another reason is that in a great many cases, the radio-frequency power supply oscillator would jump its frequency of operation before the parallel resonant point Was reached. Further, it is considered preferable to operate the load circuit when it is tuned to the inductance side of the parallel resonant point because any changes in the impedance of the load 26 per se have less efiect upon the net impedance of the load circuit and the impedance presented to the receiving end of the transmission line.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and the spirit of the appended claims.

I claim as my invention:

1. In apparatus for providing a substantially constant voltage across a variable load, the combination of a parallel-tuned load circuit including said load and a tuning impedance comprising a variable inductance connected to shunt said load, a source of oscillations at a predetermined frequency, said parallel tuned-load circuit being tunable to a parallel resonance condition at said predetermined frequency, said tuning impedance being a variable inductance and said load circuit being tuned thereby on the inductive side of said parallel resonance condition, a transmission line having a receiving end connected to said load circuit and a sending end, and a coupling circuit connected between said sending end of said transmission line and said oscillation source, said load circuit being tuned away from said parallel resonance condition of said load circuit by said tuning impedance on the inductive side of the parallel resonance condition, said transmission line having a length which is a quarter wave length or multiple thereof of said predetermined frequency, and said coupling circuit being tuned to have a relatively high impedance as compared to the sending end impedance of said transmission line.

2. The apparatus as claimed in claim 1 characterized by said coupling circuit being tuned to have a frequency for series resonance other than said predetermined frequency.

3. The apparatus as claimed in claim 1, characterized by said coupling circuit including an inductive member serially connected with a capacitive member, with said coupling circuit being in a series arrangement with said sending end of the transmission line, said coupling circuit having an efiective series impedance of said serially connected inductive member and capacitive member as presented to said source of oscillations, which series impedance is substantially greater in magnitude than the sending-end impedance of the transmission line.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,055 Wright et al. Dec. 6, 1938 2,396,708 Leeds Mar. 19, 1946 2,473,041 Urbain et al June 14, 1949 2,473,143 Craham et a1 June 14, 1949 2,523,791 Vahle Sept. 26, 1950 2,563,098 Brown Aug. 7, 1951 

